Optical phase modulators may be used in various devices including optical switches and optical modulators including for example interferometers such as a Mach-Zehnder interferometer. Phase shifts may be caused by varying the refractive index of the light transmitting medium. In the case of a semiconductor material, such as silicon, the refractive index for transmitted light may be varied by thermo-optic effect or by varying the number of charge carriers in the optical path. Silicon is used for providing optical waveguides transmitting infra red light in the spectral region above 1.1 micron wavelength. Phase modulators are known for silicon waveguides using either thermal or current injection based PIN diode arrangements. In the case of thermal arrangements, means are provided to vary the temperature of the silicon waveguides and the known thermo-optic effect is used to vary the refractive index. Such devices are too slow in operation for many applications. In the case of current injection based PIN type diodes, a semiconductor diode is formed by locating intrinsic silicon between regions of doped silicon, one region being P type and the other N type. When an electrical potential is applied to the P and N regions to forward bias the diode, the doped regions inject charge carriers into the intrinsic silicon to cause the known free carrier dispersion effect. The intrinsic silicon into which the free carriers are injected is located in the optical path of the silicon waveguide and the change in concentration of free charge carriers alters the refractive index. The time taken for the injection of free carriers as well as their recombination time is not short enough for very high speed optical switches. The recombination time may be of the order of 0.01-10.mu. seconds, and this can result in an optical switch speed no faster than 100 MHz. The requirement for switch speeds up to 1 GHz (10.sup.9 Hz) or faster is envisaged.
It is an object of the present invention to provide an improved device and method of effecting phase modulation in a silicon optical waveguide by use of carrier depletion.